1. Field of the Invention
The present invention relates to an electronic endoscope apparatus having a video-scope with an image sensor and a video-processor, and especially relates to a brightness adjustment process for a subject image displayed on a monitor.
2. Description of the Related Art
In an electronic endoscope apparatus, image-pixel signals are read from an image sensor provided at a tip portion of a video-scope, and a luminance level indicating the brightness of a displayed subject image is calculated from the image-pixel signals. Then,based on a difference between the luminance level and a reference level indicating a proper brightness of the subject image, the brightness of the subject image is adjusted so as to maintain a proper brightness. For example, the amount of light for illuminating the subject is adjusted by a diaphragm, whereas exposure time at the image sensor is adjusted by using an electronic shutter function.
Generally, to measure brightness, an average metering that calculates an average brightness or a peak metering that determines a relatively high luminance level in the subject image as a peak luminance level, is applied. The operator selects the metering method in accordance with the situation.
When the tip portion of the video-scope is extremely close to the observed portion, or when a tip portion of a metallic disposition tool, such as a forceps, extends over the tip surface of the video-scope so that a tip portion image is displayed, a so called “halation” can be generated in the peripheral area of the screen, wherein a white color image is displayed. Therefore, when the halation is generated on the peripheral area in a situation where the peak metering is applied, the brightness adjustment is performed so as to lower the luminance level even if the total brightness of the subject image is proper. One method for preventing this problem is to set the peak metering-area to an area around the center portion of the display-screen, whereas the average metering-area is set to the peripheral area.
However, because of a mucous membrane or an uneven surface of the observed portion, a luminance level of a minute area in the screen can become extremely high compared with the surrounding area. When such a minute area with a high luminance level is located at the center portion, the peak luminance level is calculated in accordance with the minute area so that the brightness adjustment process is performed so as to lower the luminance level. This adjustment process is an obstacle to proper the operation or treatment.